My Korean ARC expires at the end of August. So, a travel agent (in Korea) told me that I can only get a single entry visa to China. However, I need a double entry visa. Would it be possible for me to make a trip to Japan and obtain a double entry Chinese visa there?

My Korean ARC expires at the end of August. So, a travel agent (in Korea) told me that I can only get a single entry visa to China. However, I need a double entry visa. Would it be possible for me to make a trip to Japan and obtain a double entry Chinese visa there?

Depending on which part of China you're headed it'd likely just be cheaper to fly in via Hong Kong and do it there. Agencies can get you various visas ranging from 1 month single entry tourist to 12 month multiple entry business visas interestingly for cheaper even than the embassy charges.
Also only takes them around 3 hours as opposed to the 4 days or so most Chinese embassies usually require.